The oscillation of a falling ferrofluid droplet induced by a nonuniform magnetic field

• Published in: Experimental thermal and fluid science 2025-02, Vol.161, p.111351, Article 111351
• Main Authors: Wen, Guiye, He, Yongqing, Jiao, Feng
• Format: Article
• Language: English
• Subjects: Ferrofluids; Magnetic field; Oscillation; Viscous stress
• ISSN: 0894-1777
• DOI: 10.1016/j.expthermflusci.2024.111351

[Display omitted] •We predicted falling ferrofluid droplet deformation in a nonuniform magnetic field using a 2D model.•The applied field considerably reduces droplet oscillation and deflection.•The droplet’s oscillation frequency is proportional to the continuous phase viscosity.•Large droplet enters damped oscillation, whereas smaller one enters non-periodic attenuation oscillation. Understanding the droplet size and shape control mechanism in a magnetic field is critical for precisely manipulating ferrofluid droplets. Here, we conducted an experimental investigation on the dynamic behavior of a falling ferrofluid droplet under a nonuniform magnetic field produced by current coils. We observed an interesting phenomenon: the uneven distribution of the magnetic field and the jump in magnetic properties at fluid interfaces will cause the Laplace pressure difference on the droplet surface, stimulating the droplet’s oscillation. We also use the Laplace pressure difference equation and the interfacial tension coefficient correlation to model the deformation of ferrofluid droplets and determine the oscillation frequencies and deflection angles. The droplets’ oscillation frequency is related to the magnetic Bond number: f∼Bom-0.523∼-0.501. The deflection angle of the droplet is further diminished by the superposition of a viscous shear moment and a magnetic moment (7.41°∼12.90°). Our research lays the groundwork for precise ferrofluid droplet manipulation in drug delivery and soft robots.